The title was created due to the two brothers' spheres of influence over the spirit. Though often called by the name of dwelling places, the true names of the brothers were Námo, the elder, and Irmo, the younger.

Irmo, who resided in Lórien, was the master of visions and dreams. His gardens were the fairest in the world and filled with many spirits. Often the Valar would come to his gardens to rest and find ease from the burdens of Arda.

Námo, who resided in Mandos, had power over the fëar of Elves and Men who gathered in his halls after death, where the fëar of the Elves abode while those of Men soon left the Circles of the World. Námo delivered judgement on Manwë's bidding in serious matters (such as the Doom of the Noldor) but also judged over the fate of the elvish fëar.

Nienna is the sister of the Fëanturi, who mourned Melkor's marring of Arda. She resided west of West, near the halls of Mandos, which she often entered to bring strength and wisdom to the fëar there.[1]

Nienna and Mandos are counted among the Aratar - the eight greatest of the Valar - while their brother Irmo is not.